Superplastically formed ultrasonically welded metallic structure

ABSTRACT

Disclosed herein is a method of making a structure by ultrasonic welding and superplastic forming. The method comprises assembling a plurality of workpieces comprising a first workpiece including a first material having superplastic characteristics; ultrasonically welding the first workpiece to a second workpiece, to form an assembly; heating the assembly to a temperature at which the first material having superplastic characteristics is capable of superplastic deformation, and injecting a fluid between the first workpiece and the second workpiece to form a cavity between the first workpiece and the second workpiece.

BACKGROUND

Superplastic forming (SPF) is a fabrication technique capable of forminglarge and complex workpieces in one operation. Materials havingsuperplastic characteristics exhibit an enhanced ability to beplastically deformed without rupture, a property known assuperplasticity. This property is exhibited, for example, by certainfine-grained metals at a homologous temperature, which is a fraction ofthe material's melting point.

A typical SPF process involves placing one or more worksheets of amaterial having superplastic characteristics in a die, heating theworksheets to a temperature within the superplastic range, andsuperplastically forming the sheet(s) at the SPF temperature. Usually, adifferential forming pressure from a gas manifold is used to stretch theworksheet(s) into the desired shape against the die surface(s). Oneadvantage of SPF is that complex shapes can be formed from sheet metalso that the time and expense of milling are eliminated with great costsaving. SPF methods are also usually applicable to single and multisheetfabrication.

For multisheet fabrication, SPF is combined with joining processes toproduce sandwich structures from stacks of two or more worksheets. Forexample, combination of SPF with diffusion bonding (DB) is welldocumented and has been used in the aerospace industry for many years.Also popular is the combination of SPF/brazing, where a brazing compoundis applied where bonding is desired, SPF is carried out, and then thefaying surfaces are brazed.

However, not all materials having superplastic characteristics aresuitable for traditional joining processes. This applies, in particular,to certain alloy systems, particularly those of aluminum, magnesium, andberyllium, which do not lend themselves easily to DB or brazing. Withoutbeing bound to any theory, the main obstacle appears to be the tendencyof such metals and their alloys to form tenacious and chemically stablesurface oxide layers that interfere with the formation of ametal-to-metal contact between the faying surfaces during welding. Oxidelayers are particularly troublesome in the case of aluminum; aluminumoxide is denser than and has a melting point that is twice that of purealuminum. Accordingly, prior to welding, steps for cleaning off theoxide layer are usually needed, for example with a wire brush and/oracetone, so that the material underneath the oxide layer can be exposed.

Another disadvantage of DB relates to instances where one or more of thesheets do not react well to heat treatments, such as the protractedheating typical of DB. Moreover, DB may not be applicable in instanceswhere one or more of the sheets to be joined are heterogeneous innature, such as a sheet having a first side of a first alloycharacterized by a melting temperature of 500° C., and a second side ofa second alloy whose melting temperature is 700° C. instead. Atemperature sufficient for carrying out DB on the first side of thesheet is likely too low for the second side; conversely, a temperaturesufficient for performing DB on the second side may induce undesirablemelting of the first side. Additionally, in processes including both SPFand DB, the majority of the cycle time is taken by the DB. Typically,such processes are carried out either in a die, tying up a valuableasset, or in a dedicated furnace, which means additional equipment isneeded.

SUMMARY

These and other features, aspects and advantages of the disclosure willbecome better understood with reference to the following drawings,description and claims.

In one aspect, there is provided a method of making a structure byultrasonic welding and superplastic forming. The method comprisesassembling a plurality of workpieces comprising a first workpiececomprising a first material having superplastic characteristics;ultrasonically welding the first workpiece to a second workpiece, toform an assembly; heating the assembly to a temperature at which thefirst material having superplastic characteristics is capable ofsuperplastic deformation, and injecting a fluid between the firstworkpiece and the second workpiece to form a cavity between the firstworkpiece and the second workpiece.

In a second aspect, there is provided a multisheet structure comprisinga first sheet comprising at least one superplastically formedcorrugation and a second sheet ultrasonically welded to the first sheet.

In a third aspect, there is provided an airframe comprising at least onesuperplastically formed corrugation and a second sheet ultrasonicallywelded to the first sheet.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1D illustrate a method of manufacturing a structure byultrasonic welding and superplastic forming.

FIGS. 2A-2B illustrate a method of manufacturing a structure byultrasonic welding, structure forming, and superplastic forming.

FIG. 3 illustrates a multisheet structure featuring ultrasonic welds andcorrugations formed by superplastic forming.

FIG. 4 illustrates a schematic representation of an airframe, interior,and systems of an aircraft.

FIG. 5 illustrates an aircraft manufacturing and service method.

DETAILED DESCRIPTION

The following detailed description is of the best currently contemplatedmodes of carrying out the disclosure. The description is not to be takenin a limiting sense, but is made merely for the purpose of illustratingthe general principles of the disclosure, since the scope of thedisclosure is best defined by the appended claims.

The present disclosure is based on the discovery of a novel processcombining SPF and ultrasonic welding, hereinafter also referred to as“SPF/UW.” This process allows for the bonding and superplastic formingof metals and alloy systems difficult to join by traditional weldingtechniques, and is also applicable to joining sheets of differingcompositions. Without being bound to any particular theory, it isbelieved that the vibrations associated with ultrasonic welding break upoxide layers on the surface of workpieces, thereby making steps forremoving such layers unnecessary and promoting and facilitatingmetal-to-metal contact and therefore bonding between the fayingsurfaces. As an added advantage, by replacing lengthy and hightemperature DB steps, energy and time are saved.

Accordingly, in one aspect, there is provided a method of making astructure. A plurality of workpieces is provided, at least one of whichincludes a superplastic material. The workpieces are joined together toform an assembly; in particular, a first workpiece including asuperplastic material is ultrasonically welded to at least another,second workpiece. The assembly is heated to a temperature at which thesuperplastic material is capable of superplastic deformation, and afluid is injected between the first workpiece and the second workpiece,to form a cavity between the two. Exemplary fluids include inert gasessuch as helium, argon, and nitrogen. Different cavity sizes and shapesmay be obtained by varying the applied fluid pressure and formingcavities within a mold having a desired shape.

A representative example of the method is illustrated in FIGS. 1A-1D,where the workpieces are planar worksheets 12 and 14 placed in facingcontact with each other; worksheet 12 includes a superplastic material(FIG. 1A). The sheets may each have a respective thickness within rangesapplicable to ultrasonic bonding, usually falling within a range ofabout 0.1 mm to about 2 mm. The worksheets are then joined together byultrasonic welds 15, to form assembly 16 (FIG. 1B). Optionally, astopping off material 13, for instance yttria, boron nitride, graphite,or alumina, may be applied to one or more localized areas betweenworksheets 12 and 14 where welding is to be prevented, but this is notnormally contemplated as currently available ultrasonic joiningtechniques can be carried out with location control precise enough toselectively leave unbonded regions.

The assembly is then placed between two halves of a die 20 that arepressed together to form a gas tight seal between the edges of theassembly and the internal cavity in the die (FIG. 1C). The assembly isheated to a temperature at which superplastic forming of thesuperplastic material of worksheet 12 may take place. A fluid, such asan inert gas, is injected between worksheets 12 and 14. This causesworksheet 12 to be urged against an internal face 22 of the die 20,thereby adopting the shape of the internal face 22 (FIG. 1D). The bondsbetween the worksheets 12 and 14 include those formed by ultrasonicwelding prior to the superplastic forming. The ultrasonic welding may beapplied at individual spots or along continuous seams. In instanceswhere the assembly 16 is sealed around its outer perimeter by a weld ora bond (not shown), ducts may be included in the assembly of joinedworksheets 16, allowing a fluid to be injected into the interstitialregion between the worksheets. If necessary, gaps can be left in theultrasonic welds 15 to allow the passage of fluid between adjacent weldregions.

In instances where a product with a shape other than planar is desired,the assembly may be formed to impart a desired shape, dimensions andmechanical properties to a final product. Methods typically used for thesheet forming and the bulk forming of metals and alloys may be appliedto obtain such a product. In sheet forming, a planar precursor isdeformed by tensile loads into a three-dimensional shape, usuallywithout significant changes in thickness or surface. Typicalbulk-forming processes include forging, rolling, drawing, and extrusion.FIGS. 2A-2B illustrate an example process whereby assembly 16 is formedinto tubular piece 20 (FIG. 2A) which is in turn subjected tosuperplastic forming to yield corrugation 22 (FIG. 2B). Alternatively,the forming step may take place after the superplastic forming has takenplace.

Advantageously, ultrasonic welding does not require the hightemperatures usually associated with traditional joining methods, suchas diffusion bonding and brazing. This allows the use of materialshaving different tolerances to heat. In some examples, worksheet 14 mayinclude a material characterized by a melting point such that heatingthe sheets to temperature at which the material of worksheet 14 iscapable of diffusion bonding would result in a partial or total meltingof a material of the other worksheet 12. Since ultrasonic welding doesnot require such heating, it is suitable for joining materials havingdifferent tolerances to heat and the use of heterogeneous workpiecescomprising two or more materials having different melting points is alsomade possible. The ultrasonic welding process may be applied to blankshaving one or more sections of materials characterized by a relativelylow melting point.

In a set of representative examples, the method is applied to materialsthat do not lend themselves well to diffusion bonding. Foremost amongsuch materials are aluminum alloys, usually including magnesium,manganese, silicon, and zinc as alloying elements. Representativealuminum alloys, designated according to the International AlloyDesignation System, include the following: “1000 series” alloys with aminimum 99% aluminum content by weight; “2000 series” alloyed withcopper; “3000 series” alloyed with manganese; “4000 series” alloyed withsilicon; “5000 series” alloyed with magnesium; “6000 series” alloyedwith magnesium and silicon; “7000 series” alloyed with zinc, and “8000series” a category mainly used for lithium alloys. Also included arehigh-strength aluminum alloys finding use in aerospace applications,such as the alloys comprising aluminum, magnesium and scandium that aredisclosed in U.S. patent application Ser. No. 12/349,668 (published asU.S. 2010/0170996). Typical magnesium-based alloys include aluminum,zinc, manganese, silicon, copper, zirconium, and rare earths as alloyingelements, while common beryllium alloys include one or more of copper,cobalt, nickel, and aluminum. Also contemplated are titanium alloys, aswell as steel alloys and nickel alloys.

In another aspect, multisheet structures are provided. In suchstructures, one of the sheets has at least one superplastically formedcorrugation and is ultrasonically welded to a second sheet. The first,corrugated sheet may further be attached to the second sheet by othertypes of bonds, e.g. laser welds and/or friction welds. FIG. 3illustrates a representative example of such a structure, having outersheet 30 that is bound to inner sheet 32 by ultrasonic welds 34; outersheet 30 features superplastically-formed corrugations 36. In exemplaryembodiments, outer sheet 30 may be made of a superplastic material, suchas a 7000 series aluminum alloy, whereas inner sheet 32 may insteadinclude a material characterized by a lower superplasticity, forinstance an aluminum alloy of the 6000 series. Additional bonds betweenthe worksheets may also be present, such as laser welds and frictionwelds (not shown). Multiple layers of sheets with multiple ultrasonicwelds are also contemplated.

In representative examples, the worksheets may include materials thatusually do not lend themselves well to diffusion bonding, such asaluminum, magnesium, beryllium and their respective alloys. In otherembodiments, a sheet may include more than one material; for instance,outer sheet 30 may include an aluminum alloy and sheet 32 a magnesiumalloy or a beryllium alloy. In addition, one of the sheets may include amaterial characterized by a higher melting point than a material of theother sheet.

The above multisheet structures are ideally suited for a wide range ofapplications requiring high-strength, low-weight components. Inparticular, the structures find use in aerospace applications, e.g. inthe airframe of an aircraft 102 as shown in FIG. 4 and manufactured andserviced according to the method 100 as shown in FIG. 5. Duringpre-production, exemplary method 100 may include specification anddesign 104 of the aircraft 102 and material procurement 106. Duringproduction, component and subassembly manufacturing 108 and systemintegration 110 of the aircraft 102 takes place. Thereafter, theaircraft 102 may go through certification and delivery 112 in order tobe placed in service 114. While in service by a customer, the aircraft102 is scheduled for routine maintenance and service 116 (which may alsoinclude modification, reconfiguration, refurbishment, and so on).

Each of the processes of method 100 may be performed or carried out by asystem integrator, a third party, and/or an operator (e.g., a customer).For the purposes of this description, a system integrator may includewithout limitation any number of aircraft manufacturers and major-systemsubcontractors; a third party may include without limitation any numberof venders, subcontractors, and suppliers; and an operator may be anairline, leasing company, military entity, service organization, and soon.

As shown in FIG. 5, the aircraft 102 produced by exemplary method 100may include an airframe 118 with a plurality of systems 120 and aninterior 122. Examples of high-level systems 120 include one or more ofa propulsion system 124, an electrical system 126, a hydraulic system126, and an environmental system 130. Any number of other systems may beincluded. Although an aerospace example is shown, the multisheetstructures may be applied to other industries, such as automotiveapplications.

Apparatus and methods embodied herein may be employed during any one ormore of the stages of the production and service method 100. Forexample, components or subassemblies corresponding to production process108 may be fabricated or manufactured in a manner similar to componentsor subassemblies produced while the aircraft 102 is in service. Also,one or more apparatus embodiments, method embodiments, or a combinationthereof may be utilized during the production stages 108 and 110, forexample, by substantially expediting assembly of or reducing the cost ofan aircraft 102. Similarly, one or more of apparatus embodiments, methodembodiments, or a combination thereof may be utilized while the aircraft102 is in service, for example and without limitation, to maintenanceand service 116.

It should also be understood, of course, that the foregoing relates toexemplary aspects and embodiments of the disclosure and thosemodifications may be made without departing from the spirit and scope ofthe disclosure as set forth in the following claims.

What is claimed is:
 1. A method of making a structure by ultrasonicwelding and superplastic forming, the method comprising: assembling aplurality of workpieces having thicknesses of about 0.1 mm to about 2mm, the plurality of workpieces comprising a first workpiece and asecond workpiece with at least one of the first workpiece and the secondworkpiece being a heterogeneous workpiece, wherein the first workpiececomprises a first material having superplastic characteristics and afirst melting point and the second workpiece comprises a second materialhaving a second melting point higher than the first melting point;welding the first workpiece to the second workpiece solely by anultrasonic welding process, to form an assembly; heating the assembly toa temperature at which the first material having superplasticcharacteristics is capable of superplastic deformation; and injecting afluid between the first workpiece and the second workpiece to form acavity between the first workpiece and the second workpiece; wherein thefirst workpiece is a heterogeneous workpiece, the method furthercomprising friction welding a plurality of sections comprised ofdifferent materials together to form the first workpiece.
 2. The methodof claim 1, further comprising: placing the assembly in a die, and wheninjecting the fluid between the first workpiece and the secondworkpiece, urging at least the first workpiece against an internal faceof the die.
 3. The method of claim 1, further comprising heating thefirst workpiece and the second workpiece prior to or during theultrasonically welding the first workpiece and the second workpiece. 4.The method of claim 1, further comprising preventing the first workpiecefrom being ultrasonically welded to the second workpiece in at least onelocalized area.
 5. The method of claim 1, wherein the fluid is a gasselected from the group consisting of helium, argon, nitrogen, andcombinations thereof.
 6. The method of claim 1, wherein the frictionwelding is selected from the group consisting of friction stir welding,linear friction welding, and combinations thereof.
 7. The method ofclaim 1, further comprising forming the assembly, wherein the forming isselected from the group consisting of sheet forming, bulk forming, andcombinations thereof.
 8. The method of claim 7, wherein the forming iseffected prior to or following forming the cavity.
 9. The method ofclaim 1, wherein at least one of the first and second workpieces is aplanar worksheet.
 10. The method of claim 1, wherein heating theassembly to the temperature further comprises heating the assembly to atemperature at which the first material and the second material arecapable of superplastic deformation.
 11. The method of claim 1, whereinat least one of the first and second materials is selected from thegroup consisting of an aluminum alloy, a magnesium alloy, a berylliumalloy, a titanium alloy, a steel alloy, a nickel alloy, and combinationsthereof.
 12. The method of claim 1, wherein at least one of the firstand second materials comprises an alloy comprising aluminum and anelement selected from the group consisting of copper, manganese,silicon, magnesium, zinc, lithium, and combinations thereof.
 13. Themethod of claim 1, wherein at least one of the first and secondmaterials comprises an alloy comprising magnesium and an elementselected from the group consisting of aluminum, zinc, manganese,silicon, copper, zirconium, a rare earth, and combinations thereof. 14.The method of claim 1, wherein at least one of the first and secondmaterials comprises an alloy comprising beryllium and an elementselected from the group consisting of copper, cobalt, nickel, aluminum,and combinations thereof.
 15. The method of claim 1, further comprisingapplying a stopping off material to a localized area between the firstworkpiece and the second workpiece.
 16. The method of claim 15, furthercomprising preventing the first workpiece from being ultrasonicallywelded to the second workpiece with the stopping off material at thelocalized area.
 17. The method of claim 15, wherein the stopping offmaterial is selected from the group consisting of yttria, boron nitride,graphite, alumina, and combinations thereof.
 18. A method of making astructure by ultrasonic welding and superplastic forming, the methodcomprising: assembling a plurality of workpieces having thicknesses ofabout 0.1 mm to about 2 mm, the plurality of workpieces comprising afirst workpiece and a second workpiece with at least one of the firstworkpiece and the second workpiece being a heterogeneous workpiece,wherein the first workpiece comprises a first material havingsuperplastic characteristics and a first melting point and the secondworkpiece comprises a second material having a second melting pointhigher than the first melting point; welding the first workpiece to thesecond workpiece solely by an ultrasonic welding process, to form anassembly; heating the assembly to a temperature at which the firstmaterial having superplastic characteristics is capable of superplasticdeformation; and injecting a fluid between the first workpiece and thesecond workpiece to form a cavity between the first workpiece and thesecond workpiece; the method further comprising forming theheterogeneous workpiece by at least one of friction stir welding, rotaryfriction welding, or linear friction welding.
 19. The method claim 18,further comprising: placing the assembly in a die, and when injectingthe fluid between the first workpiece and the second workpiece, urgingat least the first workpiece against an internal face of the die. 20.The method of claim 18, further comprising heating the first workpieceand the second workpiece prior to or during the ultrasonically weldingthe first workpiece and the second workpiece.
 21. The method of claim18, further comprising preventing the first workpiece from beingultrasonically welded to the second workpiece in at least one localizedarea.
 22. The method of claim 18, wherein the fluid is a gas selectedfrom the group consisting of helium, argon, nitrogen, and combinationsthereof.
 23. The method of claim 18, further comprising forming theassembly, wherein the forming is selected from the group consisting ofsheet forming, bulk forming, and combinations thereof.
 24. The method ofclaim 23, wherein the forming is effected prior to or following formingthe cavity.
 25. The method of claim 18, wherein at least one of thefirst and second workpieces is a planar worksheet.
 26. The method ofclaim 18, wherein heating the assembly to the temperature furthercomprises heating the assembly to a temperature at which the firstmaterial and the second material are capable of superplasticdeformation.
 27. The method of claim 18, wherein at least one of thefirst and second materials is selected from the group consisting of analuminum alloy, a magnesium alloy, a beryllium alloy, a titanium alloy,a steel alloy, a nickel alloy, and combinations thereof.
 28. The methodof claim 18, wherein at least one of the first and second materialscomprises an alloy comprising aluminum and an element selected from thegroup consisting of copper, manganese, silicon, magnesium, zinc,lithium, and combinations thereof.
 29. The method of claim 18, whereinat least one of the first and second materials comprises an alloycomprising magnesium and an element selected from the group consistingof aluminum, zinc, manganese, silicon, copper, zirconium, a rare earth,and combinations thereof.
 30. The method of claim 18, wherein at leastone of the first and second materials comprises an alloy comprisingberyllium and an element selected from the group consisting of copper,cobalt, nickel, aluminum, and combinations thereof.
 31. The method ofclaim 18, further comprising applying a stopping off material to alocalized area between the first workpiece and the second workpiece. 32.The method of claim 31, further comprising preventing the firstworkpiece from being ultrasonically welded to the second workpiece withthe stopping off material at the localized area.
 33. The method of claim31, wherein the stopping off material is selected from the groupconsisting of yttria, boron nitride, graphite, alumina, and combinationsthereof.